1. Field of the Invention
This invention is concerned with analysis of organic additives and contaminants in plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions for which the additive concentration in the vicinity of the electrode surface is well defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (Ar) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide deposits with satisfactory properties and good leveling characteristics. The suppressor additive (also called the “polymer”, “carrier”, or “wetter”, depending on the bath supplier) is typically a polymeric organic species, e.g., high molecular weight polyethylene or polypropylene glycol, which adsorbs strongly on the copper cathode surface to form a film that sharply increases the overpotential for copper deposition. This prevents uncontrolled copper plating that would result in powdery or nodular deposits. An anti-suppressor additive (also called the “brightener”, “accelerator” or simply the “additive”, depending on the bath supplier) is required to counter the suppressive effect of the suppressor and provide the accelerated deposition within substrate recesses needed for leveling. Plating bath vendors typically provide additive solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
Acid copper sulfate baths have functioned well for plating the relatively large surface pads, through-holes and vias found on printed wiring boards (PWB's) and have recently been adapted for plating fine trenches and vias in dielectric material on semiconductor chips. The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC's) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the “Damascene” process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p. 32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved. The organic additives in the bath must be closely controlled since they provide the copper deposition rate differential required for bottom-up filling.
As the feature size for the Damascene process shrank below 0.2 μm, it became desirable to utilize a third organic additive in the acid copper bath in order to avoid overplating the trenches and vias. Note that excess copper on Damascene plated wafers is typically removed by chemical mechanical polishing (CMP) but the copper layer must be uniform for the CMP process to be effective. The third additive is called the “leveler” (or “booster”, depending on the bath supplier) and is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper plating rate. Leveler additive species tend to exert a relatively strong decelerating effect on the copper electrodeposition rate but are typically present in the plating bath at very low concentration so that their decelerating effect is weaker than that of suppressor additives. Due to their low concentration, leveler species tend to function under diffusion control.
In order to attain good bottom-up filling and avoid overplating of ultra-fine chip features, the concentrations of all three additives must be accurately analyzed and controlled. The suppressor, anti-suppressor and leveler concentrations in acid copper sulfate baths can all be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Likewise, the anti-suppressor concentration can be determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. A method for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor is described in U.S. Pat. No. 6,572,753 to Chalyt et al.
The concentration of chloride ion in acid copper plating baths must also be closely controlled (typically at a value in the 25 to 100 mg/L range) since chloride ion is essential to the functioning of the additive system. However, chloride ion specific electrodes are not suitable for use in acid copper plating baths because of the presence of interfering species (e.g., organic additives, copper ions and strong acid) that cause the electrode potential to drift with time. Another prior art method for chloride analysis involves titration with a solution of mercuric nitrate, which is a hazardous material that requires special handling and waste disposal. The colorimetric endpoint for this titration is also difficult to detect with sufficient accuracy, especially for an automated analysis system.
An alternative prior art method for chloride analysis of acid copper plating baths involves potentiometric titration with silver nitrate solution, for which the endpoint detection is readily automated and no hazardous waste is involved. However, the silver chloride precipitate produced during the titration is difficult to remove, and residues of the precipitate, or of a reducing agent (typically, sodium thiosulfate) used to dissolve it, can interfere with subsequent analyses performed in the same cell. The CVS methods used for analyses of organic additives in acid copper baths are particularly sensitive to interference from chloride and silver ions (derived from dissolution of the silver chloride precipitate) and reducing agents, which can affect the copper electrodeposition rate. Another disadvantage of the prior art potentiometric titration method is that the silver nitrate solution is decomposed by ambient light and must be handled in darkened containers and tubing, which interfere with visual inspection of the reagent delivery system. In addition, this titration method is only moderately sensitive to chloride ion.
U.S. Pat. No. 6,673,226 to Kogan et al., which is assigned to the same assignee as the present application, describes a voltammetric method for determining the chloride concentration in an acid copper plating bath from the effect that chloride ion exerts on the copper electrodeposition rate in the presence of organic additives. The procedure involves measuring a CVS rate parameter in a background electrolyte containing at least one organic additive but substantially no chloride ion, before and after addition of a predetermined volume fraction of the plating bath sample. Although it ameliorates cross-contamination and waste disposal issues compared to prior art titration approaches, this prior art voltammetric method is time consuming and still generates a waste stream of measurement solutions.
None of these prior art methods provides the sensitivity and robustness needed for analysis of chloride ion in production acid copper plating baths without the use of contaminating or hazardous chemicals. The prior art methods also tend to be time consuming. A chloride analysis method useful for industrial acid copper plating processes, particularly those employed by the electronics industry, is needed. Major considerations in this case are reductions in the analysis time and the process waste stream. A preferred chloride analysis method would be performed directly on the acid copper bath without dilution.
In principle, the current associated with chloride oxidation at a noble metal electrode in acid copper baths might be used to determine the chloride concentration. In practice, however, interference from adsorption and oxidation of organic additives and breakdown products and from the onset of oxygen evolution has been found to interfere with chloride determination via chloride electrochemical oxidation. Consequently, previous attempts to use the chloride oxidation current for chloride analysis have failed. The inventors have discovered, however, that the chloride concentration in an acid copper plating bath may be determined from the chloride oxidation current measured under controlled hydrodynamic conditions at a noble metal electrode using specific voltammetric parameters.